Use of material based on organic and/or inorganic fibres and chitosan for fixing metal ions

ABSTRACT

The invention concerns the use of a material based on organic and/or inorganic fibres and chitosan for fixing metal ions contained in an effluent. The invention is characterised in that chitosan represents between 0.01 and 20% by dry weight of fibres and its degree of deacetylation is higher than 90%.

This application is the U.S. national phase of international applicationPCT/FR02/02502 filed 15 Jul. 2002 which designated the U.S. and claimsbenefit of FR 0109949, dated 25 Jul. 2001, the entire contents of whichare hereby incorporated by reference.

The invention concerns the use of a material based on organic and/orinorganic fibres and chitosan for fixing metal ions contained in aliquid or solid effluent. Among the effluents containing metal ions inconsiderable proportions or traces thereof if a treatment has first beencarried out, the most affected effluents are those coming especiallyfrom the mining, nuclear, chemical and surface treatment industry, butalso from the agricultural industry like e.g. the liquid manure of pigsor the spreading sludges used as fertilizers and coming frompurification plants, but this list is not limitative.

In the rest of the description and in the claims, the expression“organic and/or inorganic fibres” denotes among the organic fibres,especially the cellulose fibres, the synthetic fibres e.g. of thepolyester or polyethylene, polypropylene, polyamide, polyvinyl chloridetype; the artificial fibres (for example viscose, cellulose acetate);the natural fibres (for example cotton, wool, wood pulp); the carbonfibres (possibly activated), and among the inorganic fibres especiallythe mineral fibres (for example glass fibres, ceramic fibres).

The chitosan is a deacetylation product of chitin, an element making upthe shells of the crabs, lobsters, shrimps or other crustaceans. As weknow, the chitosan has sequestering properties of the metal ions whenthe pH range is higher than 4.

The document WO 90/02708 describes a purification process of pollutedaqueous effluents, especially of effluents with high heavy metalcontent, by means of chitosan in the microcrystalline i.e. modifiedform. To be more precise, the microcrystalline chitosan is incorporatedinto a jelly-looking dispersion, which is put into contact with apolluted aqueous effluent. After stirring at a high temperature, thesequestered chitosan with the pollutant is separated from the solutionby filtration, sedimentation and centrifugation or by any convenientprocess. Further, it is shown that the chitosan has a degree ofdeacetylation over 30%. Considering the microcrystalline form of thechitosan, we can expect that the NH₂-functions thereof are implicated inthe crystalline lattice thus not available for metallic sequestering. Itfollows that the pollution removal technique described in this documentis not optimal.

The document GB-2 199 315 describes a support structure based on fibresof microbiological origin treated in an alkaline solution whereby thechitin they contain is revealed. According to the described process, thecultivation of mycelium is treated in an alkaline environment before orafter being deposited on a synthetic fibre structure of the polyester orpolypropylene type.

The document JP 08 13 2037 describes a water purifier, which associateschitosan and activated carbon in a granulated form in a ratio of 1:20.Although it has been shown that the mixture can adsorb the heavy metals,no quantitative information is given. In fact, all the results are givenin relation to the capacity of the purifier to eliminate the chlorinecontained in the town water.

The document JP 63 04 9212 describes an adsorbent filter consisting ofcellulose fibres, of diatoms or pearlite and of chitosan. The cellulosefibres and the diatoms are mixed in a ratio of 4/1-1/4 by weight, andthen some more chitosan is added, diluted in an acid solution in anamount of 10% by weight. Here the filter is used for separatingcolloidal proteins, micro particles and fungi. No information is givenconcerning the possibility to use this material for fixing heavy metals.Even though it would be used for this application, its efficiency wouldbe lower as the diatoms or the pearlite contain proteins that are ableto interact with the amine function of chitosan. In this case, thesequestering properties of the metals by the chitosan are affected.

The document EP-A-0 323 732 describes, furthermore, a composite materialbased on cellulose fibres, chitosan (in an amount of 1–99% by weight)and fatty acids (0.05–1% in relation to the weight of cellulose fibres).According to an essential characteristic, the chitosan used in thiscomposite material has a degree of deacetylation at least of 40%. Atsuch a degree, and according to this document, the chitosan allowsimproving the strength of the paper, especially the wet strengththereof. There is no reference made to the possibility of using thismaterial for fixing heavy metals present e.g. in an aqueous effluent.Even though this material would be used for such an application, itsefficiency would be low. In fact, the hydrophobic character that thefatty acid gives to the treated paper has an influence on the sorptionkinetics of water. By slowing down the diffusion of water into thesupport, the fatty acid also reduces the sequestering capacity. Thus theaccessibility of chitosan is reduced.

The document GB 2 338 477 describes a support based on cellulose andchemical fibres coated with chitosan in an amount of 3–20% by weight. Noindication concerning the characteristics of chitosan is mentioned. Thesupport is used for fixing e.g. metal ions of the arsenic, iron sulphateand magnesium chloride type (Ex. 6–8). If the illustrated support has agood metal ion sequestering capacity, it is only for very low effluentflow rates and, in any case, incompatible with an industrialapplication.

The problem that the invention proposes to solve thus consists inimproving the fixation speed of the metal ions and, consequently, theflow rate of the effluent to be treated. However, the Applicant hasnoticed that the fixation speed of the metal ions could be increasedwhen a low chitosan concentration was combined with a high deacetylationdegree, higher than 90%.

Consequently, the invention concerns the use of a material based onorganic and/or inorganic fibres and chitosan for fixing metal ionscontained in an effluent, and it is characterised in that the chitosanrepresents between 0.01 and 20% by dry weight of fibres and that itsdegree of deacetylation is higher than 90%.

The Applicant has in fact noticed that by affecting the concentrationand the deacetylation degree of chitosan, the fixation kinetics of metalions could be considerably modified. If one would expect that byincreasing the number of chitosan sites available by deacetylation, thefixation capacity would be increased, then on the other hand, it was notobvious that the fixation speed would improve in the same time.

The fixation of metal ions contained in an effluent can have severalapplications. First of all, the fixation of heavy metals contained in aliquid effluent can be mentioned. The material is thus used in this casefor filtration of liquid effluents by licking or running throughdepending on the degree of pollution of the effluent. The second matterreferred to is the fixation of metal ions present in the soil,especially after a chemical treatment based e.g. on copper, of anagricultural surface. In this case the material, which can correspond toa mulching paper, is used for fixing the metal ions and consequently foravoiding that they are introduced into the ground water level. Finally,the metal ions can fix themselves voluntarily on the material of theinvention either for enhancing its conducting properties, or for forminga metal layer by reduction of the fixed metal ions, or for making it amaterial having biocide properties.

In an advantageous embodiment, the chitosan represents a degree ofdesacetylation higher than 95%.

In practice, the chitosan has a molecular weight of between 10⁴ and 10⁶g.mol⁻¹, preferably between 10⁵ and 5.10⁵ g.mol⁻¹.

In an advantageous embodiment, the chitosan represents between 0.01 and10%, preferably between 0.01 and 5%, most preferably between 0.01 and 2%by dry weight of the fibres.

As already said, the material is based on organic and/or inorganicfibres, but it can advantageously consist of cellulose fibres.

According to the invention, the material is in the form of a fibrousmat, which can be manufactured according to different processes wellknown by those skilled in the art.

Thus, in a first embodiment, the chitosan is mixed with the organicand/or inorganic fibres and then a sheet is formed in a paper makingway.

In a second embodiment, a sheet is prepared of organic and/or inorganicfibres and then the sheet thus formed is impregnated by means of achitosan solution, especially with a size-press. One or two sides can beimpregnated before spin drying.

In a third embodiment, the sheet based on organic and/or inorganicfibres is coated with a chitosan solution by coating technique used in apaper mill.

The material of the invention can also be in the form of fibresuspension, especially of cellulose treated with chitosan incorporatedinto a filter cartridge.

In practice, the chitosan is used initially in the form of salt(acetate, hydrochloride etc.).

The invention finds an especially advantageous application in thetreatment of drinking water, of effluents coming especially from themining, nuclear, chemical and surface treatment industry, but also fromthe agricultural industry like e.g. the liquid manure of pigs or thespreading sludges used as fertilizers and coming from the purificationplants, but this list is not limitative.

The invention and the advantages which stem therefrom will become moreapparent from the following illustrative examples.

The FIG. 1 shows the fixation kinetics of copper by only chitosan andthe material of the invention at a copper concentration of 0.3125 mg/l.

The FIG. 2 shows the fixation kinetics of copper by only chitosan andthe material of the invention at a copper concentration of 0.625 mg/l.

The FIG. 3 shows the fixation kinetics of copper by only chitosan andthe material of the invention at a copper concentration of 3.125 mg/l.

The FIG. 4 shows the influence of the proportion of chitosan in thematerial of the invention with regard to the fixation of copper ions.

EXAMPLE 1

In this example, the fixation kinetics of copper is compared to onlychitosan and to the material of the invention, the mass of the chitosanbeing 50 mg, in static conditions, that is without an effluent fluxcoming through the filter.

a/ Chitosan

The chitosan used has a molecular weight of 200 000 g/mol and a degreeof deacetylation of 98%.

b/ Material of the Invention

A sheet based on cellulose fibres is coated with the chitosan solutionby a coating technique, whereby a support containing 2% of chitosan byweight is obtained.

c/ Protocol

-   -   only chitosan or chitosan coated on a paper is hydrated during        12 hours in 100 ml of NaNO₃-effluent (0.03 M) at a pH of 6.5,    -   a volume V of a copper mother solution is added,    -   the supernatant is taken at different moments,    -   the measure is done by I.C.P. (Inductive Coupled Plasma),    -   finally, the decontamination factor, which corresponds to the        Initial Concentration/Final Concentration ratio in the effluent        is calculated.

From this factor is deduced the more or less good decontaminationfunction of only chitosan or of the material of the invention on thebasis of the following scale:

FD=1: No decontamination

FD=7/8: Good decontamination

FD>10: Very good decontamination.

The study is carried out for decreasing copper concentrations.

As can be seen from the FIG. 1, after 210 minutes, 98% of the copper isfixed on the paper coated with chitosan, as only 13% is fixed on onlychitosan for a copper concentration of 0.3125 mg/l.

The FIG. 2 shows that for a copper concentration of 0.625 mg/l, thepaper coated with chitosan has already fixed 82% of the copper, as inthe same period of time, the chitosan film has fixed only 31% of thecopper.

Finally, for a copper concentration of 3.125 mg/l, 75% of the copper isfixed on a paper coated after 360 minutes, as only 12% of the copper isfixed on only chitosan (FIG. 3).

The decontamination factors are calculated and represented in the tablebelow.

Copper concentration Material of in mg/l Only chitosan the invention6.35  Negligible 1.1 decontamination 3.175   1.07  1.43 0.635  1.2 3.50.3175 1.4 9.7

Thus it is observed that the fixation capacity of copper by the materialof the invention is higher than that of only chitosan, and this concernsall the tested concentrations. It is further noticed that the fixationkinetics of copper on only chitosan is not so rapid than for thematerial of the invention.

In fact, for a copper concentration of 3.125 mg/l, the decontaminationfactor of 1.07, obtained after 180 minutes for only chitosan, isobtained after only 4 minutes for the material of the invention.

In the same way, for a copper concentration of 0.625 mg/l, thedecontamination factor of 1.2, obtained after 90 minutes for onlychitosan, is obtained after only 8 minutes with the material of theinvention.

For a copper concentration of 0.3125 mg/l, the decontamination factor of1.4, obtained after 200 minutes with only chitosan, is obtained afteronly 20 minutes with the paper of the invention.

EXAMPLE 2

In this example, the influence of the proportion of chitosan in acellulose sheet with respect to its copper ion fixation capacity isstudied.

As can been seen from the FIG. 4, more the chitosan proportion is low,the better is the copper ion fixation capacity of the material of theinvention in the studied scale.

EXAMPLE 3

In this example, the efficiency of the filter of the invention isstudied in dynamics for a high effluent flow rate.

The filtration system used is the filter press, the most common and thesimplest equipment in the field of liquid filtration. The effluent ispurified during its passage thorough the filtering medium consisting ofa succession of filters. The number of the consecutive filters can beadapted according to the desired performances. In this study, eachfilter having a surface of 550 cm² consists of a 100 g/m² papercontaining 1% of chitosan by mass of the degree of deacetylation of 98%.

The model effluent used is a copper solution of 6.35 mg/l which passesthrough the filtering medium with a flow rate of 4 200 l/h, the volumeof the tested effluent is 10 liters.

The rate of the retained copper, depending on the number of filters usedfor constituting the filtering medium, is represented in the followingtable.

Number of filters  4  8 10 Rate of the retained 64% 67% 70% copper

With 10 filters, 70% of the copper is fixed within less than 9 seconds.

EXAMPLE 4

In this example, in the same conditions as in the example 3, theinfluence of the amount of chitosan in the paper filter forconcentrations of 1 and 5% are compared in dynamics. The degree ofdeacetylation is 98%.

Fixation rate Number Rate of the Amount of of the % of retained theretained Amount of copper on chitosan filters copper copper chitosan thechitosan 1 4 64% 41 mg 0.2 g 20% 5 4 68% 43 mg   1 g  4%

As can be seen from the table above, the lower the amount of chitosan isin the tested range, the higher the fixation efficiency is.

1. A method of fixing metal ions contained in an effluent comprising thesteps of: a) forming a fiber mixture consisting essentially of chitosanwith organic and/or inorganic fibers by bringing chitosan having adegree of deacetylation higher than 95% into communication with organicand/or inorganic fibers such that the chitosan represents between 0.01and 10% by dry weight of the fibers, b) forming a sheet by means of apaper-machine process from the fiber mixture of chitosan with organicand/or inorganic fibers formed according to step a), and c) flowing aneffluent containing metal ions into communication with said sheet andallowing the metal ions in the effluent to be fixed to the fibersthereof by means of the chitosan.
 2. The method as recited in claim 1wherein the molecular weight of the chitosan is between 10⁴ and 10⁶g.mol-1.
 3. The method as recited in claim 1 wherein chitosan representsbetween 0.01 and 2% by dry weight of fibers.
 4. The method as recited inclaim 1 wherein step a) is practiced such that the fiber mixtureconsists exclusively of chitosan with cellulose fibers.
 5. The method asrecited in claim 1 wherein step b) comprises forming a fiber suspensionof cellulose fibers treated with chitosan and incorporating the fibersuspension into a filter cartridge.
 6. The method as recited in claim 1wherein the chitosan is in the form of a salt.